No Low Side Pressure

u118224 on Wed May 23, 2012 1:03 PM

The ac stopped working a week ago, the compressor would not run. All fuses and relay are OK. A friend of mine, who is a commercial refrigeration tech, added 134a and the compressor started cycling at high idle. The compressor appeared to be cycling off the high pressure limit switch because it shut off when the high side got up to 200. During this time there was no reading on the low side and the vents never blew cold air. My friend said the system acted like it was plugged. We pulled the orifice tube/screen from the line and I bought a new replacement and will install it tonight. I broke the screen open on the old one and the tube was not plugged, so I don't think that's the problem.

After we install the new tube/screen he'll pull a vacuum and then put in the recommended amount of 134a, 3 lbs with rear ac if I remember correctly. Assuming it still doesn't have pressure on the low side, what is the next step?

Thank you.

GM Tech on Wed May 23, 2012 3:01 PM

High pressure cut-out is 425 psi-- 200 is well within reason....no low side reading means you are not communicating with low side port--not depressing valve ball, valve closed or hose not connected correctly to gage...there has to be a low side reading especially when equalized....rapid cycling usually means too little amount of refrigerant.....

-------------------------The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

u118224 on Wed May 23, 2012 3:23 PM

We'll pull vacuum on the system tonight to get all the moisture out and put in the requisite amount of freon. We'll see what happens. Thanks for the response.

mk378 on Wed May 23, 2012 3:35 PM

These are designed to cycle on low side pressure -- off at about 25 and back on around 45. If the low side really were zero, the compressor would not run, so like GM Tech said, you're not measuring properly.

It seems the pressure was zero when you started, so there must be a substantial leak.

u118224 on Thu May 24, 2012 9:30 AM

We checked the low side port last night and found that the schrader valve did not depress when the gauge fitting was attached. I had to unscrew the schrader valve about 3/4 of a turn to get it to depress with the fitting attached. Long story short, installed the new orifice tube, pulled vacuum for an hour and a half and refilled with 3 pounds of refrigerant, and it works properly. At 64 degrees ambient, we had 30 pounds on the low side and 150 on the high side at high idle.

I agree with mk378 that there is a leak, but it's probably small. My friend has a refrigerant leak detector; I checked all the front and rear fittings, compressor fittings, around the compressor pulley and around the evaporator and no leaks were detected. The vehicle is a 2004 and this is the first time the ac has been serviced.